This invention relates to a filter for liquids such as lubricating oil, hydraulic oil and the like.
More particularly, the invention relates to a so-called spin-on filter of the type having a deep drawn sheet metal canister, a filter element within the canister, an end plate for holding the filter element in the canister, and a retaining ring for attaching the end plate to the canister. In addition to holding the filter element in the canister, the end plate provides a flow path to and from the filter element and includes means enabling the filter to be screwed onto a mounting adaptor. A filter of this general type is disclosed in Fearnhead U.S. Pat. No. 4,237,015. In the Fearnhead filter, the retaining ring is joined to the upper end of the canister by a conventional double seamed construction and coacts with the canister and the end plate to define an annular space for a preformed sealing gasket which is placed in compression during the seaming operation.
The filter of the present invention is a high strength filter which is particularly adapted for use in high pressure applications, for example, in hydrostatic transmissions. In such applications, the filter must be capable of withstanding pressure spikes in excess of 1,000 p.s.i. without bursting and must be capable of withstanding continuous cyclic pressure surges of up to 500 p.s.i. for sustained periods without fatigue failure and resulting leakage. Conventional double seamed filters fail in fatigue under high cyclic pressures due to residual stresses resulting from the seaming operation and from a hinging action between the canister and the retaining ring.
High performance filters require a large outlet port in the center of the end plate and, as a result, only a relatively small end plate area remains outboard of the outlet port, that area being perforated to form inlet ports to the filter element. In conventional filters with end plates stamped from sheet metal, the formation of the inlet ports in the relatively small available area of the end plate weakens the end plate to such an extent that it cannot withstand high pressures. As a result, prior art high strength and high performance filters generally have resorted to the use of die cast aluminum end plates. Such end plates require high initial tooling costs, high costs for maintaining the tooling, relatively long manufacturing cycle rates and significant secondary machining. In addition, a specially designed shell is required to effect fastening of a die cast end plate to the shell.